Born Margaret Jackson in Ashton-under-Lyne, she became an engineering apprentice and qualified as a metallurgist at UMIST. She worked in metallurgy and as a researcher for the Labour Party and Granada Television. She was elected as MP for Lincoln at the October 1974 general election and became a government whip and then a junior minister before losing her seat at the 1979 election.
Out of Parliament, she won election to Labour's National Executive Committee in 1980, and supported left-winger Tony Benn for the Labour leadership in 1981. Returning to the House of Commons for Derby South at the 1983 election, she gradually moved away from the hard left, supporting incumbent leader Neil Kinnock against Benn in 1988. By this time she was a front bencher, rising to Shadow Cabinet rank in 1989 as shadow Chief Secretary to the Treasury. In 1992 she became Deputy Leader of the Labour Party and shadow Leader of the House of Commons. She was acting party leader after the death of John Smith in 1994, but placed third behind Tony Blair and John Prescott in the subsequent leadership election.
Under Tony Blair she shadowed the Secretary of State for Health and then the Secretary of State for Trade and Industry, taking on the latter position after the 1997 Labour landslide. She became Leader of the House of Commons in 1998 and moved to the newly formed Defra after the 2001 election. She has managed to remain part of Blair's inner circle despite close links with the trade union movement. In August 2002 she expressed reservations about the prospect of a war in Iraq, but supported the 2003 Iraq war when the time came.